Our property was listed with booking.com and now I am facing a debt collection threat. I used their online management system to put in some special 'deals' & in doing so, and unbeknown to me, the weeks I already had blocked off as booked, were made available again. The next day I had notification of a booking - for a week that I already had guests. I notified the person who had made the booking via booking.com and booking.com immediately. Booking.com said I would still have to pay their commission, I asked why & said no on principal. They have been hounding me ever since and now threatening me with debt collectors. I have found several similar complaints on the net about this bullying after their corrupt use of their online management system.
I stood up to City Guide in a similar situation, a similar bully. I took the letter to the Gemeinde, who wrote an official letter to cityguide saying I did not have the funds, and that was the end of the matter.
This was a scammer, but booking.com is a legal business, so this is where my present problem lies.
I have replied to booking.com's email threat asking for the address to send a registered letter of complaint and asking for an official letter of reply from them. I do not expect a response though, just a visit from the debt collectors after the 14 day deadline is up.
Absolutely everyone says just pay up, but it just feels like giving in to bullies.
Any advice on which official citizens advice bureau I could contact for advice would be welcome.

Normally, when a case is sent to a debt collector, they only expect to see a small portion of the debt in return. The debt collector keeps a large portion for themselves. They then open a case with the Betribungsamt, which you can contest. Then you have hearings, and multiple meetings about the matter, then a decision.

This is time consuming and costs money. Economically and sensibly, it would not be worth it for them to pursue if the money they gain does not cover their cost for pursuing it. Hence most cases stop when you contest it at your Gemeinde. So this also depends on how much they are trying to collect from you.

Nevertheless, you really ought to look for an amicable resolution, and hold up your end of the agreement. If you owe them, then pay. If they are just trying to cheat you don't pay. But review your agreement, and settle correctly. Its not worth such money gained to live like a weasel. It will eat away at your self-respect and reputation with Booking.com.

But how to proceed depends heavily on if the debt collector knocking on your door is just a from a private debt collection agency or an official from the Office de Pursuit/Betreibungsamt. The later can seize your assets and freeze your accounts if you do not take the appropriate next steps.